The duo arrived together at last Tuesday's memorial service for Ted Branson (Richard Branson's father) at the Royal Geographical Society in London. The Daily Mail reports that Ned, 33, legally changed his name to "Ned Rocknroll" several years back, and recently separated from his wife Eliza, the daughter of a British viscount.

"Ned is really happy with Kate," the source tells Us, adding that the couple are currently spending a romantic weekend in the English countryside. Mildred Pierce star Winslet is "mature and has it all together," the source explains. "[Ned] feels he can learn so much from her. It's sexy -- a boy toy situation!"

An old friend of the Branson family, Winslet was famously a guest on Richard Branson's private Necker Island in late August when the house she was staying in caught fire; Winslet ended up carrying Eve, Branson's elderly mother, out of the burning building.

Winslet wasn't a solo visitor during her momentous Necker Island stay. In addition to her kids Mia, 10, and Joe, 7, the star's fellow guest was Louis Dowler, her on-and-off model beau.